The present invention lies in the field of determining and optimising processing steps on manufacture of sheet metal formed parts, for example by way of deep-drawing or stretch-forming processes. It relates to a method, to a data processing system, to a computer program and to a data carrier for determining a model of a geometry of a forming stage in a computer-aided design (CAD) system, according to the preamble of the respective independent patent claims.
Sheet metal formed parts as a rule are manufactured by way of deep-drawing. The semi-finished product, the so-called sheet metal blanks (or simply “blanks”) are placed into multi-part forming tools for this purpose. The parts are formed by way of presses in which the forming tools are clamped. The parts, as a rule, are manufactured from a flat sheet metal blank via several forming stages by way of machining steps such as drawing, reshaping, setting, etc., combined with trimming steps. With regard to this process, it is the edge regions, in particular the so-called addendums which represent problem zones. With regard to the design of the tools for a forming step, amongst other things, it is the case of complementing the suitably prepared component geometry, or an intermediate geometry with multi-step processes, (hereinafter both called component geometry), by an addendum in the edge zones, such that a tool geometry arises from this, with which the predefined component geometry may be manufactured such that no failure (cracks or wrinkles) occurs and that other demands on quality, for example a limited thickness reduction, achieving a minimal stretching of the sheet metal, and restrictions with regard to manufacturing geometry are adhered to.
The dimensioning and setting of the addendums represents considerable problems today. It is not rare that several months pass before a tool functions in a satisfactory manner. It is often the case of an iterative process which entails many reject parts and a high consumption of energy and resources. The production of addendums nowadays is largely effected in a manual and in an extremely time-consuming manner by way of computer-aided design systems (CAD). Such CAD-systems model a geometry of physical bodies, in particular, thus, of formed parts in various stages of processing, and of corresponding tools. Thereby, hundreds of individual surfaces are produced and edited due to the design of curves, supporting surfaces derived from these, and their trimming. Even the creation of an addendum for a large car body part may take several weeks without further measures. This procedural manner demands the designer to have a large expert knowledge in the fields of forming technology and CAD.
As an example and in detail, one proceeds as follows:
With the development of the method plan, which means determining the forming operations, and what and in which operation one forms and cuts, nowadays, one usually proceeds as follows. A simple sequence of processing steps is shown in FIGS. 2 to 8 as a basis for the explanation. FIG. 2 shows a sheet metal part or a blank 1 in an initial condition. The sheet metal blank 1 is held between a lower die 11 and a binder 10 in FIG. 3. The lower die 11 and the binder 10 as well as other holders and tools are not drawn in the following Figs. FIG. 4 shows the sheet metal blank 1 after deep-drawing, FIG. 5 after the trimming, FIG. 6 after reshaping, FIG. 7 after flanging the flange 5 and FIG. 8 after the setting of the flange. The individual conditions of a formed part are also called forming stages.
The drawing operation is firstly designed proceeding from the component geometry present in the CAD. This usually encompasses:                determining flange regions which are not formed in the draw stage,        determining a drawing position which is free of undercuts,        filling holes,        developing the binder surface and the addendum,        blunting/simplifying (sharp-edged) geometry details, in order to permit the deep-drawing,        bending over/embossing individual regions in order to compensate the springback.        
The addendum (and binder surface) is examined, for example by way of                control of the cutting angles and shear angles,        control of the size of the smallest possible blank,        examination of the forming ability and the resulting component quality by way of a simulation method,        determining trailing edges by way of a comparison of section lengths in individual section planes or by way of tracking material points in the simulation.        
Simulation in this context is to be understood as a simulation of a forming or machining process, which takes into account the physical properties of an object. The properties of a process are likewise taken into account, such as frictions, lubrication and machining speed. For example finite element methods, finite differences, boundary elements methods or so-called meshless methods are used in the simulation.
CAD-systems, thus, model static conditions in different machining stages. Thereby, the model of each condition is produced essentially manually from the basic elements, or from another model by way of modification. In contrast to this, the simulation simulates a dynamic process, or a transition between conditions whilst taking into account and/or computing physical properties such as stresses, extensions, cutting forces, strengthening, etc. CAD-systems and simulation systems are implemented today as separate program systems. An interaction at best takes place by way of a data exchange of geometry data.
An iterative change/adaptation of the addendum is carried out should individuals of the above mentioned examination criteria not be met.
A design of the following forming operations then follows by way of evaluating:                working directions,        active surfaces of the reshaping tools for the finishing forming of the geometry details,        active surfaces of the flanging/setting tools for forming the flange regions,        trimming operations (cutting directions and trimming sequence).        
From this, there results an examination of a                feasibility of the trimming lines by way of geometric analysis; and of a        forming ability and component quality by way of forming simulation.        
Follow-up operations are determined in an analogous manner in an iterative procedure. If results criteria are not met, the geometry and/or method parameters are iterative adapted proceeding from the drawing operation.
The method plan is then present, thus the regulations as to how and in which forming steps and trimming steps the component is to be manufactured.
The design of the tool bodies, casting models, production of CNC-milling data and the manufacture of the tools is effected after the release of the method plan.
The try-out is then effected, thus, the trial and initiation of the tools. This initiation is time-consuming and expensive and may last for months. One must adapt the process parameters and the tools (e.g. enlarging radii, changing regions by way of grinding-away or welding on material etc. (small iteration loops), if the desired quality and dimensional accuracy of the components is not achieved. The methods must be adapted in the extreme case (large iteration loop). In particular the correct trimming tools, i.e. the trimming line, which after further forming leads to the dimensionally correct component edge, are iteratively determined by way of trial and error.
The number of necessary try-out loops depends mainly on the quality of the method plan, in particular on the design of the drawing operation at the beginning of the forming and thus on the addendum.
The above described procedural manner for producing and examining the addendum has the following grave disadvantages:                the unrolling of the flanges, the examination of the cutting angle, the evaluation of the smallest blank and the evaluation of the trailing edges is effected via 2D-sectional analyses in a purely geometric manner, for example via a length of sectional unrollings, possibly with heuristic correction factors. First, this is inaccurate, since the material extensions caused by the forming may not be taken into account or only in an inaccurate manner, and secondly it is incomplete since the sectional analyses may not be carried out at all locations along the component edges, or the handling of the existing CAD-tools, which are not fully mature, does not yet permit a complete analysis.        If one uses physical methods instead of purely geometrical ones, thus simulation, although the accuracy is increased, the examination however is then effected in a strict sequential manner and in different programs, thus after the addendum has been modified, and the simulation is set up, carried out and evaluated in a separate step. Every examination thus becomes a more time-consuming step due to this, which often leads to the fact that only the purely geometrically methods are applied instead of the accurate physical methods.        
For this reason, methods are to be provided which simplify the working manner with a CAD-system for the preliminary design of forming parts and corresponding tools, and which improve the quality of the produced parts. The goal accuracy of the design is also to be improved, so that the effort is reduced with regard to the try-out.